The present invention relates to a liquid crystal display device, and in particular to a surface mode LCD such as a pi-cell device or a splay-bend device (SBD). The application also relates to a method of manufacture of these devices. The invention also relates to a substrate having one region of high pre-tilt and another region of lower pre-tilt, and to a method of manufacture of such a substrate.
The term xe2x80x9csurface mode LCDxe2x80x9d as used herein means a LCD in which the optical change caused by varying the electric field across the liquid crystal layer occurs primarily in the surface layers of the liquid crystal. Examples of surface mode LCDs are the pi-cell and the splay-bend device, although other types of surface mode LCDs are known. Surface mode LCDs are disclosed in xe2x80x9cSov. J. QExe2x80x9d, 1973, Vol 3, p78-79.
The pi-cell (otherwise known as an xe2x80x9coptically compensated birefringent devicexe2x80x9d or OCB) is described in xe2x80x9cMol. Cryst. Liq. Cryst.xe2x80x9d, 1984, Vol 113, p329-339, and in U.S. Pat. No. 4,635,051. The structure of a pi-cell is schematically illustrated in FIG. 1. The device comprises transparent substrates 1, 1xe2x80x2 on which are disposed alignment layers 2, 2xe2x80x2. A layer of nematic liquid crystal 3 is disposed between the substrates 1, 1xe2x80x2.
The alignment layers 2, 2xe2x80x2 create parallel alignment of the liquid crystal molecules in the liquid crystal layer 3 at its boundaries with the alignment layers 2, 2xe2x80x2. This can be achieved by using parallel-rubbed polyimide alignment layers.
Addressing electrodes (not shown) are provided on the substrates 1, 1xe2x80x2, so that an electric field can be applied to selected areas of the liquid crystal layer. The liquid crystal layer 3 is placed between linear polarisers 4, 4xe2x80x2, whose transmission axes are crossed with one another and are at 45xc2x0 to the optic axis of the liquid crystal layer.
A retarder 5, with its optic axis parallel to the optic axis of the liquid crystal layer, may optionally be provided to compensate for the retardation of the liquid crystal layer. The retarder lowers the required range for the operating voltage by allowing zero retardation of the LCD to be achieved at a finite voltage across the liquid crystal layer.
The principle of operation of the pi-cell device is illustrated in FIGS. 2(a) to 2(c).
When no electric field is applied across the liquid crystal layer, the liquid crystal is in an H-state (homogenous state), in which the liquid crystal molecules in the centre of the liquid crystal layer are substantially parallel to the substrates. This is shown in FIG. 2(a). The short lines in the figures represent the director of the liquid crystal molecules.
When an electric field greater than a threshold value is applied across the liquid crystal layer, the liquid crystal molecules adopt a V-state (or a bend state). In this state, the liquid crystal molecules in the centre of the liquid crystal layer are substantially perpendicular to the substrates. FIG. 2(b) shows a first V-state which occurs at a low applied voltage across the liquid crystal layer, and FIG. 2(c) shows a second V-state which occurs when a higher voltage is applied across the liquid crystal layer. The pi-cell is operated by switching the liquid crystal layer between the first, low voltage V state and the second, higher voltage V-state.
If the electric field across the liquid crystal layer should be reduced below the threshold value, the liquid crystal layer will relax to the H-state of FIG. 2(a); in order to re-commence operation of the device, it is necessary to put the liquid crystal layer back into the V-state. This generally requires a large applied voltage, owing to the low pre-tilt of the liquid crystal molecules. The pre-tilt is usually below 45xc2x0 and typically between 2 and 10xc2x0 so as to provide sufficient optical modulation and fast switching between the two V-states (for instance of the order of a millisecond or less).
One problem with known OCB devices is the difficulty of nucleating and stabilising the V-state, which is topologically distinct from the H-state. One prior art technique is described in UK Patent Application 9521043.1. In this prior technique, the V-state is nucleated under the application of a high voltage, and is stabilised by the polymerisation of a network whilst a high voltage is applied. This prior art technique is, however, unsuitable for use in active matrix devices, since it is difficult to apply voltages having the required magnitude in a TFT panel. A further disadvantage is that the in-situ polymerisation can lead to ionic contamination of the liquid crystal layer, and result in image sticking.
The SBD device, which is also a surface mode device, is described in UK Patent Application No. 9712378.0. The structure of a SBD device is generally similar to that of a pi-cell, except that the alignment layers in a SBD device have a high pre-tilt whereas the alignment layers in a pi cell have a low pre-tilt. An SBD device uses a liquid crystal material with a negative di-electric anisotropy, whereas a pi-cell uses a liquid crystal material having a positive di-electric anisotropy.
The principle of operation of a SBD is illustrated in FIGS. 3(a) to 3(c). When no voltage is applied across the liquid crystal layer, a V-state is stable as shown in FIG. 3(a). When an electric field greater than a threshold value is applied across the liquid crystal layer, an H-state becomes stable. FIG. 3(b) shows a first H-state which occurs at a low applied voltage across the liquid crystal layer, and FIG. 3(c) shows a second H-state which occurs when a higher voltage is applied across the liquid crystal layer. In operation, the device is switched between the low voltage H-state of FIG. 3(b) and the high voltage H-state of FIG. 3(c). If the electric field across the liquid layer is reduced below the threshold value, the liquid crystal will relax into the V-state, and it will be necessary to put the liquid crystal back into the H-state before operation can be re-commenced.
The high pre-tilt alignment layers required for a SBD can be produced, for example, by the photo-polymerisation of a mixture of reactive mesogens, as described in UK Patent Application No. 9704623.9.
SID 97 Digest, page 739, discloses a method of promoting nucleation of the V-state in a pi-cell. Voltages of the order of 20 V are applied across the liquid crystal layer to switch the liquid crystal from the H-state to the V-state. However, it is difficult to provide voltages of this magnitude in a TFT (thin film transistor) substrate.
Japanese published Patent Application JP-A-9 90432 (Toshiba) discloses the provision of nucleation sites within a pi-cell panel. The nucleation sites are provided by including spacer balls or pillars within the pi-cell panel, and cooling the liquid crystal material from an isotropic phase to a nematic phase while an electric field is applied across the panel. This results in some of the spacer balls/pillars acting as nucleation sites for growth of the V-state into the H-state. This prior art has a number of disadvantages. Firstly, it requires additional process steps during fabrication of the panel, since it is necessary to align the liquid crystal molecules under the influence of an applied electric field. These additional process steps complicate the fabrication of the panel. Secondly, some spacer balls/pillars can nucleate the H-state into the V-state thus destabilising the operating state of the panel.
Japanese published Patent Application JP-A-9 218411 discloses an LCD having a bend alignment state which is stabilised, in the absence of an applied field, by the presence of spacers in the form of spherical particles. The spacers have a surface energy such that liquid crystal molecules adjacent the alignment layers are mainly aligned parallel to the alignment layers. However, in order for this technique to work, a field has to be applied during the initial alignment of the device. Also, the particles cannot be positioned so as to be outside the pixel apertures so that the contrast ratio of the display is reduced by the presence of the particles.
Miwa et al IDW 97-Digest page 739 disclose a method of maintaining the stability of a V-state in a pi-cell. A resetting period is provided within each frame, and the high voltage V-state is addressed in this period. This prevents the liquid crystal layer relaxing to the H-state when low driving voltages are applied. This does not, however, address the initial nucleation of the V-state from the H-state.
U.S. Pat. No. 4,400,060 discloses a liquid crystal cell in which the liquid crystal is switched between an H-state and a V-state to obtain optical modulation i.e. the optical state of each pixel is defined by being either in the V-state or in the H-state. Pixels defined in the liquid crystal layer are separated from one another by a neutral isolation region, which completely separates a pixel from an adjacent pixel. The isolation region is to prevent a disclination at the boundary of one pixel drifting into an adjacent pixel and causing inadvertent switching of the adjacent pixel. The isolation regions are defined by varying the pre-tilt of one alignment film. This gives rise to a pinned disclination along the whole perimeter or boundary of each pixel.
U.S. Pat. No. 5,781,262 discloses a technique for making an LCD in which the liquid crystal layer in each pixel has domains of different orientations in order to improve the viewing angle characteristics. A polyimide orientation film is subjected to a first rubbing step, after which selected portions of the film are masked. The unmasked portions are then subjected to a second rubbing step in the opposite direction. This technique results in an alignment layer which provides very low pre-tilt angles.
U.S. Pat. No. 5,757,454 also discloses a technique for making an LCD with pixel domains having different orientations. The alignment layer is initially rubbed in a first direction and selected portions are masked. The unmasked portions are then rubbed in a different direction (not opposite the first direction) so as to define the different domain orientations. This results in different portions of the alignment layer having different directions of pre-tilt.
A first aspect of the present invention provides a liquid crystal display device comprising: a liquid crystal layer disposed between first and second substrates; and means for applying a voltage across the liquid crystal layer; wherein, when no voltage is applied across the liquid crystal layer, a first liquid crystal state is stable in a first volume defined in the liquid crystal layer and a second liquid crystal state is stable in a second volume defined in the liquid crystal layer; and wherein, when a voltage greater than a threshold voltage is applied across the liquid crystal layer, a third liquid crystal state becomes stable in the second liquid crystal volume, characterised in that the area of the first substrate corresponding to the first liquid crystal volume does not enclose the area of the first substrate corresponding to the second liquid crystal volume.
The first liquid crystal volume is acting as a nucleation region, and promotes the change of the second liquid crystal volume to the third liquid crystal state. A lower applied voltage is required to put the second liquid crystal volume in the third liquid crystal state than if the nucleation region is not provided.
The third stable state may be the same type of state as the first stable state. The first liquid crystal state may remain stable in the first volume defined in the liquid crystal when a voltage greater than the threshold voltage is applied across the liquid crystal layer.
The first stable state may be a HAN-state, the second stable state may be an H-state, and the third stable state may be a V-state. Alternatively, the first stable state may be a V-state and the second stable state may be an H-state, and the third stable state may be a V-state. The device may be a pi-cell.
The first stable state may be a HAN-state, the second stable state may be a V-state, and the third stable state may be an H-state. Alternatively, the first liquid crystal state may be an H-state and the second liquid crystal state may be a V-state. The device may be a splay-bend device.
The area of the first substrate corresponding to the first liquid crystal volume may have a different pre-tilt than the area of the first substrate corresponding to the second liquid crystal volume. The area of the second substrate corresponding to the first liquid crystal volume may also have a different pre-tilt than the area of the second substrate corresponding to the second liquid crystal volume. These are straightforward ways to define the first and second volumes in the liquid crystal region.
The first liquid crystal volume may comprise a liquid crystal material dispersed in a polymer matrix. This represents an alternative way of defining the first and second volumes in the liquid crystal layer.
The area of the first substrate corresponding to the first liquid crystal volume may be completely enclosed by the area of the first substrate corresponding to the second liquid crystal volume.
A second aspect of the present invention provides a method of operating a liquid crystal display device comprising the steps of: providing a liquid crystal display device having a liquid crystal layer; defining first and second volumes in the liquid crystal layer such that a first liquid crystal state is stable in the first volume defined in the liquid crystal layer and a second liquid crystal state is stable in the second volume defined in the liquid crystal layer when no voltage is applied across the liquid crystal layer; and applying a voltage across the liquid crystal layer such that the third liquid crystal state becomes stable in the second volume defined in the liquid crystal layer.
The third liquid crystal state may be the same type of state as the first liquid crystal state. The first liquid crystal state may be a V-state and the second liquid crystal state may be an H-state, or the first liquid crystal state may be an H-state and the second liquid crystal state may be a V-state.
The first liquid crystal state may be a HAN-state, the second liquid crystal state may be an H-state, and the third liquid crystal state may be a V-state, or the first liquid crystal state may be a HAN-state, the second liquid crystal state may be a V-state, and the third liquid crystal state may be an H-state.
It can be seen that the principle of this invention is quite different from U.S. Pat. No. 4,400,060. In this prior art document, the liquid crystal regions which do not switch when a voltage is applied are the xe2x80x9cneutral isolation regionsxe2x80x9d, and it is essential that these completely surround the active pixel areas of the liquid crystal layer. In contrast, in the present invention, the first liquid crystal regionxe2x80x94the xe2x80x9cnucleation regionxe2x80x9dxe2x80x94is not required to surround the second liquid crystal region. Indeed, the first liquid crystal region may be completely enclosed by the second liquid crystal region. In contrast with the disclosure of U.S. Pat. No. 4,400,060, the present invention ensures that, at zero applied voltage, a non-pinned disclination is present in or in near proximity to each pixel. On application of a voltage, the disclination moves over the pixel area to initialise one defined operating state, for example the V-state or the H-state, which is different from the zero voltage state. The optical modulation state of each pixel is then defined by the voltage applied to this state.
The present invention allows one or more V-state nucleation regions to be fabricated into an OCB device. Providing these nucleation regions allows the remainder of the liquid crystal region to be put into the V-state for a relatively low applied voltage, so that the device is compatible with a TFT panel. The nucleation regions are provided by varying an alignment condition, for example the surface pre-tilt, over the liquid crystal layer.
The present invention can also be applied to other types of surface mode LCDs, such as an SBD device.
A third aspect of the present invention provides a method of manufacturing a substrate comprising the steps of: (a) providing a high pre-tilt alignment layer; and (b) rubbing one or more selected areas of the alignment layer so as to reduce the pre-tilt of the rubbed area(s) and leaving at least one unselected area of the alignment layer unrubbed so as to retain the high pre-tilt.
The method may comprise the further step of (c) masking the alignment layer except for the one or more selected areas, the step (c) being carried out before the step (b).
The un-rubbed alignment layer may have a pre-tilt of substantially 90xc2x0. This will produce homeotropic alignment of a liquid crystal layer placed adjacent to the alignment layer. Alternatively, the un-rubbed alignment layer may produce a conical alignment condition.
The alignment layer may be a polymer alignment film.
A fourth aspect of the present invention provides a substrate produced by a method as defined above.
A fifth aspect of the present invention provides a liquid crystal display device comprising a substrate as defined above. The device may be a parallax barrier, or it may be a HAN device having two or more liquid crystal domains.